Feature - Reference prefixes when mapping #270

Added .Prefix() method to CsvPropertyReferenceMap so that reference
properties could have their fields prefixed if needed.

Note: Initially I used the same style as for PrefixReferenceHeaders in
CsvConfiguration, using the format [Prefix].[FieldName]. However, I
removed the '.' separator as this made it less flexible than simply
allowing the user to determine their own prefix scheme